1. Field of the Invention
This invention is in the field of controlling a cement making method in which the raw material passes from a pre-heating zone to a calcining zone and then to a sintering zone and cooling zone. The specific improvements of the present invention center around measuring the degree of deacidification of the product and using this measurement as a basis for controlling efficient operation of this system.
2. Description of the Prior Art
In a modern cement burning system, the raw meal is pre-calcined to a specific degree, up to 90% in some instances, in a heat exchanger which includes a pre-heating zone and a calcining zone. In order to improve the efficiency and retain an uncomplicated process sequence, it is highly desirable to operate the heat exchanger such that the thermally treated material leaves with a constant, predetermined degree of deacidification so that it may be completely deacidified in the rotary tubular kiln and subsequently sintered. Such a process has the advantage that additional disruptions are kept away from the regulation of the rotary kiln.
Some of the difficulties which have to be overcome lie from the fact that the condition of the burning material cannot be measured directly in terms of deacidification or in terms of clinker formation without substantial investment. As a result, the prior art has gone to so-called substitute values. There are further difficulties presented from the fact that the individual zones of the burning system have only limited accessibility. Consequently, the measuring devices can only be installed at the boundaries of the zones.
In order to overcome these difficulties, it has been proposed to select the exhaust gas as the parameter which determines the output for the system and to regulate the specific amount of raw meal introduced in relationship to that value. It is also common to meter the amount of fuel required for the burning process in relationship to the ratio of amount of exhaust gas with the amount of raw meal. The temperature of the exhaust gas thereby serves as an indirect control means for the heat exchange. An amount of raw meal is also added so that this temperature remains constant. This type of system is described in DEX Zement-Kalk-Gips-No. 4/1972, pages 164 through 166).
A further difficulty to be overcome in the control of the burning process arises from the complexity of the thermal decomposition of calcium carbonate. According to GMELINS HANDBUCH DER ANORGANISCHEN CHEMIE, 8th Edition, 1956, the decomposition temperature can range from about 812.degree. C. to 1120.degree. C. with a CO.sub.2 pressure of 760 Torr. These differences can be explained by the presence of a third, solid phase having variable additives whose occurrence depends essentially on the heating conditions (Chapter 28, Calcium-Oxide, pp. 31 ff.).
It is further known that calcination in the temperature range between 800.degree. and 1100.degree. C. proceeds at a constant velocity for each temperature. Calcination occurs in a very narrow zone, the phase boundary between CaO and CaCO.sub.3. Various substances such as steam or carbon monoxide have an accelerating effect and under certain conditions, a catalytic effect. It is further known that the temperature coefficient of the reaction kinetics in the dissociation is equal to the temperature coefficient of the equilibrium pressure. The dissocation pressure thereby rises exponentially in relationship to the temperature. This is explained from the fact that events occurring in the dissociation are comparable to the evaporation of a fluid in that the development of a gaseous phase occurs due to a chemical separation process. For example, a dissociation pressure of 0.2 Torr at a temperature of 506.degree. C. rises to 879 Torr at 904.degree. C. and to 30,000 Torr at 1420.degree. C.
The reaction kinetics of the calcium carbonate decomposition in the calcining process depends on the following, process-oriented parameters:
1. Dwell time of the material in the reaction system; PA1 2. Material temperature; PA1 3. Partial CO.sub.2 pressure of the ambient gas stream; PA1 4. Solids charge of the gas stream; PA1 5. Mean grain size of the material; PA1 6. Composition of the material; PA1 7. Influence of accelerating or retarding additives. PA1 TE.sub.u is the lowest temperature at which decarbonization of the sample begins. PA1 TE.sub.o is the maximum temperature at which decarbonization is complete. PA1 (a) a metering device for setting and detecting the mass streams of the fuels supplied to the burning system at various locations; PA1 (b) a metering means for setting and detecting the mass stream of the product supplied to the calcining zone; PA1 (c) an O.sub.2 gas analyzer at a location between the sintering zone and the feed of the combustion air to the calcining zone, as well as at the discharge of the calcining zone and, if necessary, at the exhaust gas discharge; PA1 (d) a CO.sub.2 gas analyzer, particularly at the two aforementioned locations; PA1 (e) temperature measuring means for: PA1 1. A unit for calculating the degree of deacidification which is coupled to the device used for acquiring the measured value; PA1 2. A rated value input unit; PA1 3. A rated value/actual value comparison unit; PA1 4. A control unit for setting the manipulated variable for the fuel feed to the calcining zone according to the measurement of the identified degree of deacidification; PA1 5. A controllable metering unit for feeding fuel into the calcining zone.
It has been determined that the kinetic data of the specific calcium carbonate dissociation can be experimentally identified by keeping the most important parameters constant and varying one. (DEZ Zement-Kalk-Gips; No. 2/1979, pp. 78 through 82).